PURPOSE: To manufacture a drive control system of an AC servomotor with a simple configuration and to generate a sinusoidal wave drive signal without any phase delay.

CONSTITUTION: Magnetic pole position signals U and V having waveforms similar to the induction voltage waveform of an AC servomoter are output as magnetic pole position signals from an optical encoder 2 which is mounted to an AC servomotor 1. The sinusoidal wave drive current of each phase for driving the AC servomotor is generated. A light transmission band in a curved shaped where the radius vector fluctuates in a state which is similar to the induction voltage waveform of the AC servomotor according to the rotation of a rotary circular plate is formed at the rotary circular plate of the encoder 2, thus configuring a circuit for converting information indicating a detected magnetic pole position to a sinusoidal waveform, simplifying the configuration since no software processing is required, and generating a sinusoidal wave drive signal without causing any phase delay due to software processing.